Kenya lets you do both in one trip. The order matters more than most people expect.
Kenya lets you do both in one trip — plains and reef in the same fortnight. The order matters more than most people expect.
Do the safari first. Game drives start before dawn and involve a lot of time in a vehicle on rough roads; you want to arrive at that fresh rather than sun-tired. The coast then works as the decompression at the end, which is what a beach is actually good for.
The practical route is Nairobi, out to the parks, back to Nairobi, then either the SGR train or a short flight down to Mombasa or Malindi. Allow a buffer day between the two halves — safari itineraries run long more often than they run short.